Multilayer device and method of making

ABSTRACT

The invention relates to composite articles comprising a substrate and additional layers on the substrate. According to one example, the layers are selected so that the difference in the coefficient of thermal expansion (CTE) between the substrate and a first layer on one side of the substrate is substantially equal to the CTE difference between the substrate and a second layer on the other side of the substrate. The stress caused by the CTE difference and/or shrinkage on one side of the substrate during heating or cooling is balanced by the stress caused by the CTE difference on the other side of the substrate during heating or cooling. Such stress balancing can reduce or minimize curling of the substrate.

FIELD OF THE INVENTION

The present invention relates generally to multilayer devices, and moreparticularly to a multilayer electronic device comprising a polymericsubstrate and a method of making the device.

BACKGROUND OF THE INVENTION

Various processes are known for fabricating electronic devices such asopto-electrical devices, photovoltaic devices, and liquid crystaldisplay (LCD) devices. Commonly, these devices have been fabricated witha glass substrate and a conductor applied to the substrate which servesas an electrode. The conductor is first coated onto a side of the glasssubstrate, and then one or more additional layers are provided tocomplete the device. For example, in the case of an organic lightemitting device (OLED), a transparent conductor such as indium tin oxide(ITO) may be coated onto a glass substrate to form an anode. Next, anelectroluminescent layer comprising, for example, a blend of a holetransport polymer, an electron transport polymer and a light emissivepolymer may be formed on the anode. Finally, a cathode is formed on theelectroluminescent layer. The process of applying one or more of thelayers may comprise steps that are carried out at an elevatedtemperature to achieve improved device properties.

One advantage of glass substrates is their low permeability to oxygenand water vapor, which reduces corrosion and other degradation of theOLED device. However, glass substrates are not suitable for certainapplications in which flexibility is desired. In addition, manufacturingprocesses involving large glass substrates are typically slow and cantherefore result in high manufacturing costs.

Recently, plastic substrates have been used in the fabrication ofelectronic devices. Plastic substrates have advantages over glasssubstrates because of their flexibility, light weight, thinness, androbustness. However, there can be certain technical challenges infabricating electronic devices on plastic substrates. For example, thefabrication temperature typically must remain below the glass transitiontemperature, Tg, of the plastic substrate so that the substratemaintains its desirable physical properties, such as flexibility andtransparency. In addition, plastic substrates typically have arelatively high coefficient of thermal expansion (CTE) compared toinorganic layers which may be applied in the fabrication process. Amaterial's CTE indicates its expansion and contraction properties as afunction of temperature. Furthermore, plastic substrates shrink afterheating at elevated temperatures. Unlike thermal expansion, shrinkage isgenerally irreversible. Thermal expansion combined with shrinkage cantherefore cause the article to curl significantly during heating andcooling processes, which may pose significant challenges duringmanufacturing.

Known electronic devices with plastic substrates typically have anotherdisadvantage relating to oxygen and moisture diffusion. For example,plastic substrates are generally not impervious to oxygen and watervapor, and thus may not be suitable for the manufacture of certaindevices such as OLEDs which may benefit from such properties. In orderto improve the resistance of these substrates to oxygen and water vapor,coatings comprising ceramic materials have been applied to a surface ofthe plastic substrate. However, the interface between polymeric andceramic layers is typically weak due to the incompatibility of thematerials, and the layers are prone to be delaminated.

Accordingly, there is a need to provide flexible electronic devices thatare robust against degradation due to environmental elements. There isalso a need for reducing or preventing the stress and curl which mayresult from manufacturing processes employing thin film materials withvarying CTEs.

SUMMARY OF THE INVENTION

According to one embodiment, the invention relates to an articlecomprising a polymeric substrate having a first side and a second side,a first layer on the first side of the polymeric substrate, the firstlayer having a first coefficient of thermal expansion (CTE), a secondlayer on the second side of the polymeric substrate, a third layer onthe second layer, the third layer comprising a transparent conductor,and a fourth layer on the third layer, the fourth layer comprising anorganic semiconductor layer, wherein the second layer has a second CTEwhich is substantially equal to the first CTE.

The invention also relates to a method of making a multilayer articlecomprising the steps of applying a first layer on a first side of apolymeric substrate, the first layer having a first coefficient ofthermal expansion (CTE), applying a second layer on a second side of thepolymeric substrate, heating the polymeric substrate, applying a thirdlayer comprising a transparent conductor on the second layer, coolingthe article, and applying a fourth layer on the third layer, the fourthlayer comprising an organic semiconductor layer, wherein the secondlayer has a second CTE which is substantially equal to the first CTE.

According to yet another embodiment, the invention relates to an articleand a method for making the article, wherein article comprises acomposite substrate comprising a first plastic substrate, a secondplastic substrate, and a layer between the first plastic substrate andthe second plastic substrate, wherein the composite substrate has afirst coefficient of thermal expansion (CTE), and a transparentconductor on the composite substrate, the transparent conductor having asecond CTE, wherein the first CTE is substantially equal to the secondCTE.

According to still another embodiment, the invention relates to anarticle and a method for making the article, wherein the articlecomprises a polymeric substrate, a first layer on one side of thepolymeric substrate, the first layer having a first coefficient ofthermal expansion (CTE), and a second layer on the other side of thepolymeric substrate, the second layer having a second CTE, wherein thesecond layer comprises a transparent conductor, and the first CTE issubstantially equal to the second CTE.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram showing the curvature of a coated substrate;

FIG. 2 is a graph of the CTE of a composite substrate as a function oflayer thickness, illustrating one aspect of the present invention;

FIG. 3 is a photograph illustrating the curvature of a first articleresulting from a high-temperature fabrication process on a plasticsubstrate where one side is coated with ITO as compared with a secondarticle resulting from the same process where both sides are coated withITO;

FIG. 4 is a photograph illustrating the different curvatures resultingfrom a fabrication process on a plastic substrate where one side of thesubstrate is coated with a fixed thickness of ITO and the other side iscoated with varying thicknesses of SiNx;

FIG. 5 illustrates one embodiment of the present invention;

FIG. 6 illustrates another embodiment of the present invention;

FIG. 7 illustrates another embodiment of the present invention;

FIG. 8 is a flow chart showing a method of producing the article of FIG.5 according to an exemplary embodiment of the invention;

FIG. 9 is a flow chart showing a method of producing the article of FIG.6 according to an exemplary embodiment of the invention; and

FIG. 10 is a flow chart showing a method of producing the article ofFIG. 7 according to an exemplary embodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 is a diagram illustrating the curvature of a substrate 11 coatedwith a layer 10. The curling of the substrate 11 can result, forexample, from heating a plastic substrate such as polycarbonate (PC)having a relatively high coefficient of thermal expansion (CTE),applying a transparent conductor such as indium tin oxide (ITO) having alower CTE to the substrate, and then cooling the composite article. Asshown in FIG. 1, R is the radius of curvature measured from the centerof curvature to the inner edge 12 of the substrate 11, d is thesubstrate thickness, l is the length of the substrate 11, and θ is theangle defining the endpoints of the length l of the substrate 11. Whentwo flat materials curl, the relationship between the length l of thesubstrate, the shrinkage percent (Δl/l), and the radius of curvature Rcan be described by the following equations:Δ1=θ·(R+d)−θ·R=θ·d

-   -   Shrinkage percent=Δl/l=θ·d/[θ·(R+d)]    -   R=d·(Δ1)

In general, as the radius R increases, the curl decreases. As theshrinkage percent of the substrate decreases, the radius R increases andthe curl decreases. A low shrinkage percent will result in a largeradius R and small curl. A high shrinkage percent will result in a smallradius R and large curl. The thinner the substrate 11, the morepronounced the curl. Reducing the CTE difference between the layer 10and the substrate 11 results in an increased radius and reduced curl.

FIGS. 5-7 illustrate embodiments of the invention which can be madeaccording to the methods described in FIGS. 8-10, respectively.

FIG. 5 shows an article comprising a substrate 50, a first layer 51, asecond layer 52, a third layer 53, and a fourth layer 54. The substrate50 may comprise a plastic material such as polycarbonate or anotherpolymer such as polyethyleneterephthalate, polyacrylate, silicone, epoxyresin, silicone-functionalized epoxy resin, polyester, polyimide,polyetherimide, polyethersulfone, polyethylenenapthalene, polynorbonene,or poly(cyclic olefin), for example. One particular example of asuitable material for the substrate 50 is a polycarbonate with a1,3-bis(4-hydroxyphenyl)menthane repeat unit (1,3-BHPM PC, or BHPM). Thesubstrate 50 is typically thin and flat and may be transparent oropaque. According to one embodiment, the substrate 50 has a thickness ofabout 76 microns (3 mils).

The first and second layers 51, 52 may comprise, for example, atransparent low-CTE material, such as a metal nitride, metal oxide,metal oxy-nitride, or any combination thereof. Examples include aluminumoxide, aluminum nitride, aluminum oxy-nitride, silicon oxide, siliconnitride, silicon oxy-nitride, cadmium oxide, indium oxide, tin oxide,and mixtures thereof. If desired, these materials can be doped withaluminum, nitrogen, fluorine, carbon, boron, phosphor, indium, and/orhydrogen, for example. According to exemplary embodiments of theinvention, one or both of the first and second layers 51, 52 comprise(s)silicon nitride (SiNx), silicon oxy-nitride (SiNxOy), or an amorphoushydrogenated silicon nitride (a-SiNx:H), where x may range from 0 to 2and y may range from 0 to 2, for example. According to anotherembodiment the first and second layers 51, 52 comprise tin-doped indiumoxide (ITO). The CTE value of these materials may range from 1 to 20ppm/K, more typically ranges from 1 to 10 ppm/K, and most typicallyranges from 5 to 10 ppm/K.

The first and second layers 51, 52 described above can function asgas/moisture barrier layers, for example, to prevent or diminish oxygenand water from passing into or through the coated substrate 50. Agas/moisture barrier layer may have a transmission rate of oxygenthrough the coated substrate of less than approximately 0.1 cm³/(m²day), as measured at 25 degrees C. and with a gas containing 21volume-percent oxygen, and a transmission rate of water vapor throughthe coated substrate of less than approximately 1 g/(m² day), asmeasured at 25 degrees C. and with a gas having 100-percent relativehumidity, for example.

In one embodiment the first and second layers 51, 52 comprise the samematerial, such as SiNx. In another embodiment, the layers 51, 52comprise additional layers of coatings and substrates, such as anadditional substrate-coating-substrate structure.

FIG. 5 also shows a third layer 53 which may comprise a transparentconductor (TC), for example. As shown in FIG. 5, the third layer 53 canbe applied to the second layer 52. Examples of suitable materials forthe third layer 53 include transparent conductors such as thin metalsand transparent conducting oxides, for example. A particular example isindium tin oxide (ITO). The third layer 53 may function as an electrode.

The article of FIG. 5 also shows a fourth layer 54. The fourth layer 54may comprise an electroluminescent layer, for example. Examples of asuitable material for an electroluminescent layer include a polymer, acopolymer, a mixture of polymers, or lower molecular-weight organicmolecules having unsaturated bonds. Such materials possess a delocalizedπ-electron system, which gives the polymer chains or organic moleculesthe ability to support positive and negative charge carriers with highmobility. Suitable electroluminescent polymers arepoly(N-vinylcarbazole) (“PVK”, emitting violet-to-blue light in thewavelengths of about 380-500 nm); poly(alkylfluorene) such aspoly(9,9-dihexylfluorene) (410-550 nm), poly(dioctylfluorene)(wavelength at peak electroluminescent emission of 436 nm), orpoly{9,9-bis(3,6-dioxaheptyl)-fluorene-2,7-diyl} (400-550 nm);poly(praraphenylene) derivatives such as poly(2-decyloxy-1,4-phenylene)(400-550 nm). Mixtures of these polymers or copolymers based on one ormore of these polymers and others may be used to tune the color ofemitted light.

Another class of suitable electroluminescent polymers is thepolysilanes. Polysilanes are linear silicon-backbone polymerssubstituted with a variety of alkyl and/or aryl side groups. They arequasi one-dimensional materials with delocalized σ-conjugated electronsalong polymer backbone chains. Examples of polysilanes arepoly(di-n-butylsilane), poly(di-n-pentylsilane), poly(di-n-hexylsilane),poly(methylphenylsilane), and poly{bis(p-butylphenyl)silane} which aredisclosed in H. Suzuki et al., “Near-Ultraviolet ElectroluminescenceFrom Polysilanes,” 331 Thin Solid Films 64-70 (1998). These polysilanesemit light having wavelengths in the range from about 320 nm to about420 nm.

Organic materials having molecular weight less than about 5000 that aremade of a large number of aromatic units are also applicable. An exampleof such materials is 1,3,5-tris{n-(4-diphenylaminophenyl)phenylamino}benzene, which emits light in the wavelength range of380-500 nm. The electroluminescent layer also may be prepared from lowermolecular weight organic molecules, such as phenylanthracene,tetraarylethene, coumarin, rubrene, tetraphenylbutadiene, anthracene,perylene, coronene, or their derivatives. These materials generally emitlight having maximum wavelength of about 520 nm. Still other suitablematerials are the low molecular-weight metal organic complexes such asaluminum-, gallium-, and indium-acetylacetonate, which emit light in thewavelength range of 415-457 nm,aluminum-(picolymethylketone)-bis{2,6-di(t-butyl)phenoxide} orscandium-(4-methoxy-picolylmethylketone)-bis(acetylacetonate), whichemits in the range of 420-433 nm.

The article of FIG. 5 can form part of an electronic device. Forexample, a second electrode can be applied to the fourth layer 54 toform an OLED device. In this case, when a voltage is supplied by avoltage source and applied across the first and second electrodes, theelectroluminescent layer emits light. The articles of FIGS. 6 and 7 canalso similarly form part of an electronic device, as will be appreciatedby those skilled in the art.

A method of making the article shown in FIG. 5 will now be describedwith reference to FIG. 8. First, in optional step 80, the substrate 50is heated. Preferably, the substrate 50 is heated to a temperature belowthe glass transition temperature, Tg, of the substrate 50. The substrate50 may be heated to 200 degrees Celsius or more, for example.

In steps 81 and 82, the first layer 51 and the second layer 52 areapplied to the substrate 50. These steps 81, 82 may occur in reverseorder, or they may occur simultaneously. The steps 81, 82 may begin theprocess, or they may occur after the substrate 50 is heated in optionalstep 80. The first and second layers 51, 52 may be in a heated orunheated state when they are applied to the substrate 50.

The first and second layers 51, 52 may be applied to the substrate 50using a variety of known methods. For instance, some compositions suchas SiNx may be deposited on the substrate by plasma-enhanced chemicalvapor deposition (PECVD). Other compositions, such as ITO, may besputter-deposited on the substrate. Generally, the coating materials,including transparent conductors, may be applied to the substrate by amethod such as plasma-enhanced chemical-vapor deposition,radio-frequency plasma-enhanced chemical-vapor deposition, expandingthermal-plasma chemical-vapor deposition, sputtering, reactivesputtering, electron-cyclotron-resonance plasma-enhanced chemical-vapordeposition, inductively-coupled plasma-enhanced chemical-vapordeposition, expanding thermal-plasma chemical-vapor deposition,radio-frequency plasma-enhanced chemical-vapor deposition, orcombinations thereof.

If the substrate 50 is not heated in step 80, then the composite articlecan be heated in step 83. If the substrate 50 is heated in step 80, thenthe composite article need not be heated in step 83. Preferably, thearticle is heated to a temperature below the glass transitiontemperature, Tg, of the substrate 50.

Because of the CTE difference between the substrate 50 and the first andsecond layers 51, 52, heating the composite article may create stressforces between the substrate 50 and the first layer 51 on one side ofthe substrate 50, and also between the substrate 50 and the second layer52 on the other side of the substrate 50.

According to exemplary embodiments of the invention, the first andsecond layers 51, 52 are selected so that the first layer 51 on one sideof the substrate 50 stress balances the second layer 52 on the otherside of the substrate 50 during a cooling and/or heating process. Forexample, the difference in CTE between the substrate 50 and the firstlayer 51 can be chosen to be substantially equal to the CTE differencebetween the substrate 50 and second layer 52. The CTE-induced stressforces on one side of the substrate 50 substantially balance thecorresponding stress forces on the other side during the heatingprocess. Such stress balancing can reduce or minimize curling of thesubstrate during heating.

While the composite article is in a heated state (either from step 80 or83), a third layer 53, which may comprise a transparent conductor, isapplied to the second layer 52 in step 84. The third layer 53 maycomprise a thin metal and/or transparent conducting oxide, for example.According to one embodiment, the third layer 53 comprises ITO. The ITOmay contain approximately nine times more indium than tin, by mass, forexample. According to one embodiment, the third layer 53 functions as anelectrode, and the heating step is useful for applying the third layer53 onto the second layer 52 and/or improving the electro-opticalproperties of the third layer 53.

In step 85, the composite article is cooled. In a preferred embodiment,the stress forces resulting from the CTE differential between thesubstrate 50 and the first layer 51 on one side of the substratesubstantially balance the corresponding stress forces resulting from theCTE differential between the substrate 50 and the second layer 52, onthe other side of the substrate during the cooling process. Theresulting curl of the composite article is consequently reduced orminimized.

After the first, second and third layers 51, 52, and 53 are applied tothe substrate 50, any desired additional layers, such aselectroluminescent layers and a second electrode for an OLED, can beapplied to complete the electronic device.

According to another embodiment, the three layers 51, 52, 53 areselected so that the first layer 51 on one side of the substrate 50stress balances the combination of the second and third layers 52, 53 onthe other side of the substrate 50 during a cooling and/or heatingprocess. However, the CTE-induced stress resulting from the third layer53, which may comprise a transparent conductor for example, is typicallysmall or negligible compared to that of the first and second layers 51,52. In such case, only the first and second layers 51, 52 may need to beselected to balance the CTE-induced stress from heating and/or coolingsince the CTE-induced stress from the third layer 53 is relativelysmall.

FIG. 6 illustrates an article according to another embodiment of theinvention. The article of FIG. 6 can be fabricated using the methodillustrated in FIG. 9. The article comprises a first substrate 61, asecond substrate 62, a first layer 63 between the substrates, and atransparent conductor layer 64 on the first substrate 61.

The first and second substrates 61, 62 may comprise a plastic materialsuch as polycarbonate or another polymer such aspolyethyleneterephthalate, polyacrylate, silicone, epoxy resin,silicone-functionalized epoxy resin, polyester, polyimide,polyetherimide, polyethersulfone, polyethylenenapthalene, polynorbonene,or poly(cyclic olefin), for example. One particular example of asuitable material for the substrates 61, 62 is a polycarbonate with a1,3-bis(4-hydroxyphenyl)menthane repeat unit (1,3-BHPM PC, or BHPM). Thesubstrates 61, 62 are typically thin and flat. According to oneembodiment, each of the substrates 61, 62 has a thickness ofapproximately 25-250 microns. Typically, the first substrate 61 istransparent. The second substrate 62 may be transparent or opaque.

The first layer 63 between the two substrates 61, 62 may comprise atransparent low-CTE material, such as a metal nitride, metal oxide,metal oxy-nitride, or any combination thereof, for example. According toone embodiment of the invention, the first layer 63 comprises SiNx.According to another embodiment, the first layer 63 comprises ITO. Asdescribed above with respect to the first and second layers 51 and 52 inFIG. 5, the first layer 63 may function as a gas/moisture barrier layerto prevent or diminish oxygen and water from passing into or through thecoated substrate.

The transparent conductor layer 64 may comprise a thin metal and/ortransparent conducting oxide, for example. According to one embodiment,the transparent conductor layer 64 comprises ITO. The ITO may containapproximately nine times more indium than tin, by mass, for example. Thetransparent conductor layer 64 may function as an electrode, forexample.

The first substrate 61, the second substrate 62, and the first layer 63together form a what may be referred to as a composite substrate. Thecomposite substrate may have a CTE which is a function of the CTEs ofits individual layers. The article may be designed such that the CTE ofthe composite substrate is substantially equal to the CTE of thetransparent conductor layer 64.

The article in FIG. 6 can be formed using the method illustrated in FIG.9. Referring to FIG. 9, in optional step 90, the first substrate 61 isheated, as described above in connection with FIGS. 5 and 8. If thefirst substrate 61 is not heated in this step, the first substrate 61may be heated later in step 93.

In step 91, the first layer 63 is applied to a first side of the firstsubstrate 61. The first layer 63 may be applied by any of the methodsdescribed above with reference to FIGS. 5 and 8. This step may begin theprocess of FIG. 9, or it may occur after the first substrate 61 isheated.

In step 92, a second substrate 62 is laminated or otherwise affixed tothe first layer 63. The first and second substrates 61, 62 may be in aheated or unheated state during step 92. Typically, the first layer 63and the first and second substrates 61, 62 are selected so that thecomposite substrate (61, 62, 63) has a relatively low CTE. As will bedescribed further below, the graph in FIG. 2 illustrates how such acomposite substrate may be designed to have a relatively low CTE, e.g.,by increasing the thickness of the first layer 63.

In optional step 93, if the first substrate 61 was not heated in step90, the composite article is heated. If the composite article has a lowCTE, it exhibits relatively little curling during the heating stepbecause it does not expand or contract significantly.

The components 61, 62, 63 of the composite substrate can also beselected such that that the CTE-induced stress between the first layer63 and the first substrate 61 balances the stress between the firstlayer 63 and the second substrate 62 in a heating and/or coolingprocess.

The composite substrate can be designed with one or both of thesefeatures, e.g., the composite substrate itself can have a relatively lowCTE and/or the CTE-induced stress on each side of the first layer 63 isbalanced.

During the heat treatment, the transparent conductor layer 64 is appliedto the uncoated side of the first substrate 61 in step 94. This heatingstep may improve the properties of the transparent conductor layer 64.For example, the heating step may effect a reduction in sheetresistance, increased bulk conductivity, and/or increased lighttransmission in the wavelength range of 400 to 700 nanometers of thetransparent conductor layer 64.

Finally, in step 95, the resulting article is cooled. During the coolingstep, curling of the resulting article can be reduced or minimized. Thecurling can be reduced because the two materials affixed to each other,namely the transparent conductor layer 64 and the composite substrate ofstep 92, both have low CTEs. In addition, the curling can be reduced dueto stress balancing on opposite sides of the first layer 63. Inaddition, the curling can be reduced because of stress balancing betweenthe composite substrate and the transparent conductor layer 64. In manycases, the effect of a CTE-induced stress of the second layer 64 isinsignificant compared to the stress forces of the other components.Thus, as long as the composite substrate 61, 62, 63 is designed tominimize or reduce curling, then the article including the second layer64 also will minimize or reduce curling, because the effect of thesecond layer 64 is relatively small.

FIG. 7 illustrates an article according to another embodiment of theinvention comprising a first layer 71, a second layer 72, and asubstrate 73. FIG. 10 illustrates a method of making the article shownin FIG. 7. The first and second layers 71, 72 and the substrate 73 maybe formed of the materials and by the methods described above inconnection with FIGS. 5-6 and 8-9. According to one embodiment, thefirst layer 71 comprises SiNxOy or SiNx, the second layer 72 comprises atransparent conductor such as ITO, and the substrate 73 comprisespolycarbonate.

Referring to FIG. 10, in optional step 100, the substrate 73 is heated,as described above in connection with FIG. 8. If the substrate 73 is notheated in this step, the substrate 73 may be heated later in step 102.

In step 101, the first layer 71 is applied on one side of the substrate73. This step may occur after heating in optional step 100, or it may bethe first step in the process of FIG. 10. In optional step 102, thecomposite article is heated if the substrate 73 was not heated in step100. There may be a CTE differential between the substrate 73 and firstlayer 71 which results in some curling during this heating step 102.

Steps 103 and 104 provide for an optional cooling and then re-heatingprocess. Thus, the method of FIG. 10 can pass from step 102 straight tostep 105, or it can pass from step 102 through both steps 103 and 104 tostep 105. In step 103, the composite article is cooled. As a result ofthis cooling, the CTE differential may cause significant curling. Atthis point, the composite article may be collected and stored before itundergoes further fabrication. If desired, the article can be flattenedprior to storage and prior to step 104. In step 104, the compositearticle is heated again.

In step 105, the second layer 72, preferably a transparent conductor, isapplied to the other side of the substrate 73 using one of the methodsdescribed above. The article now has two low-CTE layers 71, 72 on eitherside, which can balance stress forces that might otherwise result fromcooling. In step 106, the article is cooled. The first and second layers71, 72 can be selected so that the article is stress-balanced duringcooling, thereby minimizing or eliminating curling.

It should be appreciated, however, that some small CTE differentialmight be desirable or advantageous in any of the above embodiments. Insuch a case, the layers and substrate(s) can be selected to achieve thedesired CTE differential and/or curl. It should also be appreciated thatany stress-balancing contemplated above will depend on the chemicalproperties and dimensions (e.g., thickness, length, width, anduniformity) of the layers and substrates.

EXAMPLE 1

FIG. 3 is a photograph illustrating two composite articles resultingfrom a heat treatment of a plastic substrate coated with ITO on one side(article on the left) or both sides (article on the right). The plasticsubstrate comprised 1,3BHPM polycarbonate film having a thickness of 75microns. The ITO had a thickness of 150 nanometers. The ITO was dcmagnetron sputtered from a 10% SnO₂, 90% In₂O₃ sputtering target. Thesputtering was conducted at room temperature and the heat treatment wasconducted at 200° C. Before heat treatment, the articles had a flat,circular shape with a diameter of approximately 17.8 centimeters (7inches) and a thickness of 75 microns. The photograph of FIG. 3 showsthe two articles after heat treatment. The article on the left side ofthe photograph has one side coated with ITO. This article showssignificant curvature after heat treatment, as the originally flatplastic material has curved to such an extent that it has rolled into acylindrical shape having a radius of less than 2.54 cm (1 inch). This isbecause the shrinkage of the plastic material created a substantialresidual stress causing the material to curl after heating. On the rightside of the photograph is a plastic substrate having both sides coatedwith ITO. The article is substantially flat after heat treatment. Thisis because the stress resulting from the shrinkage of the plasticsubstrate is balanced by the stress resulting from the shrinkage on theother side of the substrate.

EXAMPLE 2

FIG. 2 is a graph showing the calculated CTE of a hypothetical compositearticle as a function of layer thickness according to an exemplaryembodiment of the invention. In this embodiment, the hypotheticalcomposite article comprises a silicon nitride (SiNx) layer sandwichedbetween two 76.2 micrometer (3 mil) layers of a polycarbonate substrate.The data in FIG. 2 are based on theoretical calculations using thematerial parameters from a-SiNx:H and 1,3-BHPM polycarbonate.

In general, the CTE of a composite article depends on the respectiveCTEs of its constituent parts as well as the relative proportions ofthose constituent parts. Polycarbonate has a relatively high CTEcompared to silicon nitride. A composite article consisting of acombination of polycarbonate and silicon nitride will typically have acomposite CTE between the two CTEs of its constituent parts with a valuedependent on the relative proportions of those parts.

FIG. 2 shows that as the relative proportion of low-CTE SiNx increasesin the composite article, the CTE of the composite article decreases toapproach the CTE of the SiNx. When the thickness of the SiNx layer iszero, the CTE of the polycarbonate is approximately 70 ppm/K. When thethickness of the SiNx layer is 10 nanometers (nm), the composite CTEdecreases to approximately 25 ppm/K. When the thickness of the SiNxlayer is 20 nm, the composite CTE decreases further to approximately 15ppm/K. Thus, increasing SiNx thickness can significantly reduce the CTEof the composite article. As described above in connection with FIG. 6,the CTE of the composite substrate can be matched to the CTE of anotherlayer for the purpose of stress balancing.

EXAMPLE 3

FIG. 4 is a photograph illustrating articles with different curvaturesresulting from a fabrication process on a plastic substrate where oneside of the substrate is coated with a fixed thickness of ITO (150 nm)and the other side is coated with varying thicknesses of SiNx. Theplastic substrate comprised 1,3-BHPM polycarbonate film having athickness of 75 microns. The x in the SiNx ranged from 0.8 to 1.4. TheSiNx was plasma deposited in a parallel-plate, capacitively-coupledplasma fed with silane, ammonia and helium. The ITO was dc magnetronsputtered from a 10% SnO₂, 90% In₂O₃ sputtering target. The SiNx wasapplied at a temperature of 75 degrees C. and the ITO was applied at atemperature of 75 degrees C. Before application of any of the coatingsat elevated temperature, each article had the shape of a thin and flatsquare with a length of approximately 10.2 centimeters (4 inches). Thefar left article shows the control article, which had no SiNx layer (0nm). In this article, the stress forces resulting from the CTEdifference between the plastic substrate and the ITO layer caused asignificant curl. Because the other side of the plastic substrate had nolayer, there was no opposite stress from a CTE imbalance to prevent ordiminish the curl. One edge of the article has substantially curled backonto its opposite edge so that the article as a whole has a cylindricalshape.

The article second from the left has a layer of 70 nm of SiNx on theside opposite to the 150 nm ITO layer. In this article there is curling,but the curl is less pronounced than the article with no SiNx layer. Thearticle third from the left was coated with 100 nm of SiNx. While thereis some curling, the article maintains a relatively flat shape, and thecurl is much less pronounced than in either of the aforementionedarticles. In the last article, on the right side of the photograph, the150 nm ITO layer is balanced with a 150 nm SiNx layer on its oppositeside. In this article, the stress resulting from the CTE differencebetween the plastic and ITO layer is balanced by the stress resultingfrom the CTE difference between the plastic and the SiNx layer. Thisexample illustrates advantages which can be provided by an embodiment ofthe article shown in FIG. 7.

It will be understood that the specific embodiment of the inventionshown and described herein is exemplary only. Numerous variations,changes, substitutions and equivalents will now occur to those skilledin the art without departing from the spirit and scope of the presentinvention. Accordingly, it is intended that all subject matter describedherein and shown in the accompanying drawings be regarded asillustrative only and not in a limiting sense and that the scope of theinvention be determined by the appended claims.

1. An article comprising: a polymeric substrate having a first side anda second side; a first layer on the first side of the polymericsubstrate, the first layer having a first coefficient of thermalexpansion (CTE); a second layer on the second side of the polymericsubstrate; a third layer on the second layer, the third layer comprisinga transparent conductor; and a fourth layer on the third layer, thefourth layer comprising an organic semiconductor layer; wherein thesecond layer has a second CTE which is substantially equal to the firstCTE.
 2. The article of claim 1, wherein the fourth layer comprises anorganic electroluminescent layer.
 3. The article of claim 1, wherein thefourth layer comprises an organic photovoltaic layer.
 4. The article ofclaim 1, wherein said transparent conductor comprises a metal or aconducting oxide.
 5. The article of claim 1, wherein the transparentconductor comprises indium tin oxide.
 6. The article of claim 1, whereinthe polymeric substrate comprises polycarbonate.
 7. The article of claim1, wherein the polymeric substrate comprises at least one of:polyethyleneterephthalate, polyacrylate, silicone, epoxy resin,silicone-functionalized epoxy resin, polyester, polyimide,polyetherimide, polyethersulfone, polyethylenenapthalene, polynorbonene,and poly(cyclic olefin).
 8. The article of claim 1, wherein thepolymeric substrate comprises at least one of:polyethyleneterephthalate, polyethelenenapthalene, polynorbonene, andpoly(cyclic olefins).
 9. The article of claim 1 wherein the polymericsubstrate comprises a polycarbonate with a1,3-bis(4-hydroxyphenyl)menthane repeat unit, the second and thirdlayers comprise SiNx, and the third layer comprises indium tin oxide.10. The article of claim 1, wherein the first and third layers eachcomprise at least one of: a metal nitride, a metal oxide, and a metaloxy-nitride.
 11. The article of claim 1, wherein the first and secondlayers comprise the same material in substantially equal amounts. 12.The article of claim 1, wherein at least one of the first layer and thesecond layer comprises a gas/moisture barrier coating which causes atransmission rate of oxygen through the substrate of less than about 0.1cm³/(m² day) as measured at 25 degrees C. and with a gas containing 21volume-percent oxygen, and a transmission rate of water vapor throughthe substrate of less than about 1 g/(m² day) as measured at 25 degreesC. and with a gas having 100-percent relative humidity.
 13. The articleof claim 1, wherein the article forms part of an electronic device. 14.An article comprising: a composite substrate comprising a first plasticsubstrate, a second plastic substrate, and a layer between the firstplastic substrate and the second plastic substrate, wherein thecomposite substrate has a first coefficient of thermal expansion (CTE);and a transparent conductor on the composite substrate, the transparentconductor having a second CTE, wherein the first CTE is substantiallyequal to the second CTE.
 15. The article of claim 14, wherein thecomposite substrate has a CTE of less than 20 ppm/K.
 16. The article ofclaim 14, wherein the composite substrate has a CTE of less than 5ppm/K.
 17. The article of claim 14, wherein the transparent conductorcomprises a metal or a conducting oxide.
 18. The article of claim 14,wherein the first and second plastic substrates comprise polycarbonate.19. The article of claim 14, wherein the layer between the first plasticsubstrate and the second plastic substrate comprises at least one of: ametal nitride, a metal oxide, and a metal oxy-nitride.
 20. A multilayerarticle comprising: a polymeric substrate; a first layer on one side ofthe polymeric substrate, the first layer having a first coefficient ofthermal expansion (CTE); and a second layer on the other side of thepolymeric substrate, the second layer having a second CTE, wherein thesecond layer comprises a transparent conductor, and the first CTE issubstantially equal to the second CTE.
 21. The multilayer article ofclaim 20, wherein the polymeric substrate comprises polycarbonate. 22.The multilayer article of claim 20, wherein the first layer comprisesSiNxOy, wherein x ranges from 0 to 2 and y ranges from 0 to
 2. 23. Themultilayer article of claim 20, wherein the second layer comprises atransparent conductor.
 24. A method of making a multilayer articlecomprising the steps of: applying a first layer on a first side of apolymeric substrate, the first layer having a first coefficient ofthermal expansion (CTE); applying a second layer on a second side of thepolymeric substrate; heating the polymeric substrate; applying a thirdlayer comprising a transparent conductor on the second layer; coolingthe article; and applying a fourth layer on the third layer, the fourthlayer comprising an organic semiconductor layer; wherein the secondlayer has a second CTE which is substantially equal to the first CTE.25. The method of claim 24, wherein the step of heating the substrate iscarried out before applying the first and second layers.
 26. The methodof claim 24, wherein the step of heating the substrate is carried outafter applying the first and second layers.
 27. The method of claim 24,wherein the first and second layers are applied to the first substrateby one of the following methods: plasma-enhanced chemical-vapordeposition, radio-frequency plasma-enhanced chemical-vapor deposition,expanding thermal-plasma chemical-vapor deposition, sputtering, reactivesputtering, electron-cyclotron-resonance plasma-enhanced chemical-vapordeposition, inductively-coupled plasma-enhanced chemical-vapordeposition, expanding thermal-plasma chemical-vapor deposition, andradio-frequency plasma-enhanced chemical-vapor deposition.
 28. Themethod of claim 24, wherein the first and second layers are applied tothe first substrate by at least one of: plasma-enhanced chemical vapordeposition and sputtering.
 29. The method of claim 24, wherein thefourth layer comprises an organic electroluminescent layer.
 30. Themethod of claim 24, wherein the fourth layer comprises an organicphotovoltaic layer.
 31. The method of claim 24, wherein the polymericsubstrate comprises polycarbonate.
 32. The method claim 24, wherein thefirst and third layers each comprise at least one of: a metal nitride, ametal oxide, and a metal oxy-nitride.
 33. The method claim 24, whereinat least one of the first layer and the second layer comprises agas/moisture barrier layer which causes a transmission rate of oxygenthrough the substrate of less than about 0.1 cm³/(m² day) as measured at25 degrees C. and with a gas containing 21 volume-percent oxygen, and atransmission rate of water vapor through the substrate of less thanabout 1 g/(m² day) as measured at 25 degrees C. and with a gas having100-percent relative humidity.
 34. A method of making a multilayerarticle comprising: applying a first layer on a first side of a firstpolymeric substrate; applying a second polymeric substrate to the firstlayer, wherein the first and second polymeric substrates and the firstlayer comprise a composite substrate having a first coefficient ofthermal expansion (CTE); heating the substrates; applying a second layerto a second side of the first polymeric substrate, the second layercomprising a transparent conductor and having a second CTE which issubstantially equal to the first CTE; and cooling the article.
 35. Themethod of claim 34, wherein the first layer, the first polymericsubstrate, the second polymeric substrate, and the second layer areselected such that after the cooling step, the article has a radius ofcurvature greater than or equal to 10 centimeters.
 36. The method ofclaim 34, wherein the heating step occurs before the step of applyingthe first layer.
 37. The method of claim 34, wherein the heating stepoccurs after the step of applying the first layer.
 38. The method ofclaim 34, wherein: the first layer comprises at least one of a metalnitride, a metal oxide, and a metal oxy-nitride; and the first andsecond polymeric substrates comprise polycarbonate.
 39. A method ofmaking a multilayer article comprising: applying a first layer on afirst side of a polymeric substrate, the first layer having a firstcoefficient of thermal expansion (CTE); heating the polymeric substrate;applying a second layer to a second side of the polymeric substrate, thesecond layer comprising a transparent conductor and having a second CTEwhich is substantially equal to the first CTE; and cooling the article.40. The method of claim 39, wherein the first layer and the second layerare selected such that after the cooling step, the article has a radiusof curvature greater than or equal to 10 centimeters.
 41. The method ofclaim 39, further comprising the following additional three steps, priorto the heating step: heating the polymeric substrate; cooling thepolymeric substrate, wherein the cooling results in significant curlingof the substrate; and storing the resulting article comprising the firstlayer and the first substrate.
 42. The method of claim 39, wherein theheating step occurs before the step of applying the first layer.
 43. Themethod of claim 39, wherein the polymeric substrate comprisespolycarbonate and the first layer comprises SiNxOy, wherein x rangesfrom 0 to 2 and y ranges from 0 to 2.